Yog-Sothoth (or the Great Intelligence) was a disembodied sentience who attempted to find a body and physical existence.
Yog-Sothoth was a being who existed before the present universe. It was a member of a race of beings called the Great Old Ones, who were the equivalent of the Time Lords in their native realm. They shunted themselves into a parallel dimension to pass into the current Universe. Yog-Sothoth discovered he had gained god-like powers. He was the military strategist of his people.He decided to see if various stratagems he had considered would work and waged a campaign on a million inhabited worlds. Yog-Sothoth was under pressure from the other Great Old Ones, who themselves embarked on similar campaigns and conquered other planets. (MA: Millennial Rites)
At some point, the Great Intelligence possessed the Tibetan lama Padmasambhava while he was travelling the astral plane and forced him to build its Robot Yeti over the next centuries. (DW: The Abominable Snowmen)
In the 19th century, the Intelligence made a parallel attempt to destroy Earth, manifesting as living snow that used Walter Simeon as a host whose dark thoughts powered it. He established the G.I. Institute as the snow slowly swarmed to Earth over 50 years, and in 1892 was sufficient enough to consume mankind.
Having erased Simeon's mind, the Eleventh Doctor, unaware he was dealing with the Intelligance, was surprised to see the Intelligence survived, thinking it was created by the host Simeon, but it had learned to survive beyond physical form. It was stopped in the last minutes of Christmas Eve when the snow was drowned by the tears of Captain Latimer's family crying at Clara Oswald's death. During this encounter, the Doctor showed the Intelligence a schematic of the London Underground. (DW: The Snowmen)
The Travers family
In 1935, the Second Doctor, with Edward Travers, a westerner determined to find the Yeti, intervened. Padmasambhava's physical body died when the Intelligence melted away. (DW: The Abominable Snowmen)
Thirty-five years or so later, the robot Yeti re-activated and the Intelligence manifested as webbing. It ensnared the Doctor's TARDIS in space and forced it to land in the London Underground. Reunited with Travers, the Doctor assisted British military in their battles with the Yeti. The Intelligence re-animated and possessed the corpse of Staff Sergeant Arnold, using him to track the Doctor's actions. The Intelligence captured the Doctor and tried to use a conversion headset to take over the Doctor's body. The Doctor attempted to reverse the process, allowing him to absorb the Intelligence and destroy it. When the control spheres that formed the focus of the Intelligence were smashed by Jamie McCrimmon, the Intelligence vanished, powerless but still alive. (DW: The Web of Fear)
Anne Travers, left traumatised by the Intelligence's first attempts to enter the current universe, was left believing millionaire Ashley Chapel would try to use a special program, the Millennium Codex, to summon the Intelligence to Earth. He prepared a counterspell to force it back into its own reality. However, this had destructive effects, including merging the Intelligence and the benevolent god Saraquazel into a single malevolent being. It altered reality around London to a realm partly obeying the laws of the Intelligence's universe. This was undone, and the Intelligence was thrust back in its prison (MA: Millennial Rites) the bubble universe. (DW: The Doctor's Wife)
The Intelligance, renaming itself House to draw in more prey, lived in a small bubble universe outside the main universe, existing as an intelligence in a time rift and its body was an asteroid-sized celestial object.
The Intelligance feasted on artron energy and at some point discovered other Time Lords, whose TARDISes were filled with just such a power source. It began using Gallifreyan distress signals to lure Time Lords to its surface, where they would be trapped. The Intelligance would remove the TARDIS' matrix and devour the TARDIS. The trapped Time Lords would perish.
Uncle, Auntie, the Ood Nephew and Idris came upon the Intelligance and lived on its surface. As Time Lords died there, he renewed the bodies of his servants and replaced parts of their dying bodies with dead Gallifreyans who had also stumbled upon the Intelligance. They became an extension of his will and carried out his every wish.
After the near-annihilation of the Time Lords in the Last Great Time War, the Intelligance grew hungry and attracted the attention of the Eleventh Doctor, who was accompanied by Amy Pond and Rory Williams, by sending a psychic distress signal box, originally broadcast by a Time Lord called the Corsair, into their universe through the rift.
The Intelligance used his powers to remove the Doctor's TARDIS' matrix and put it into the body of Idris, leaving it blank for it to be consumed. However, during a conversation with the Doctor, who was unable to recognise "House" as the Greate Intelligance, it learnt the Time Lords had perished in a terrible conflict. Realising its food source had been eliminated, the Intelligance decided to change its behaviour by possessing the now empty TARDIS and using it to reach the main universe to find new sources of energy. The Doctor found the cache of hypercubes sent by the dead Time Lords the Intelligance had trapped. The Doctor and Idris built a TARDIS from the scraps of old TARDISES about the asteroid to pursue the Intelligance.
To amuse itself, the Intelligance used the TARDIS to play with the minds of Amy and Rory. However, the Doctor caught up with the Intelligance. The TARDIS matrix was released from her dying human body, allowing her to repossess the time ship. The Intelligance's essence was expunged from the ship. (DW: The Doctor's Wife)
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- The Seventh Doctor, Lysandra Aristedes and Sally Morgan encountered the Great Intelligence during their travels in the black TARDIS. (BFA: Black and White)
The Intelligence thought very highly of itself, informing the Doctor that his brain was too small to grasp its purpose. (DW: The Abominable Snowmen) Without a body, it became obsessed with having physical form, craving symmetry of light, colour, and shape. (DW: The Web of Fear)
It designed invasions based on a "Great Plan" it had, having been the military strategist of the Old Ones, and when it thought resistence was useless for humanity, it asked Brigadier Lethbridge-Stewart which part of its brilliant plan he found most effective. (RP: Downtime) When horribly hound to Saraquazel, it did not care about him or crave his assistance, thinking it possessed enough power and intellect to find its own escape. The Intelligence, however, did warn Saraquazel of the duplicity of human beings. (MA: Millennial Rites)
Habits and Quirks
The Great Intelligence had no physical existence. This meant it was incapable of interacting with the rest of the universe. It could possess living creatures such as humans and could use control spheres to form a locus for the Intelligence. (DW: The Abominable Snowmen) Also, it manifested as webbing to trap the Doctor's TARDIS. (DW: The Web of Fear)
In the universe before this one, Yog-Sothoth had a body that was a mass of tentacles and mouths. (MA: Millennial Rites) Like the other Old Ones, it may have lost its body when it ran from the Fendahl. (NA: White Darkness) Afterwards, the Intelligence constantly sought physical existence to replace being a shapless, formless cloud hanging in space. (DW: The Web of Fear)
The Intelligence possessed several people as its main instruments, namely Padmasambhava (DW: The Abominable Snowmen), Staff Sgt. Arnold (DW: The Web of Fear), Edward Travers (RP: Downtime), and Walter Simeon (DW: The Snowmen). It also used Auntie and Uncle to speak when it first met the Doctor, Rory and Amy. The green glow in the eyes of Ood Nephew was presumably also an indicator of the Intelligance's direct telepathic influence. (DW: The Doctor's Wife)
When not using a living being, it maintained a basic manifestation as a three-sided pyramid of control spheres (DW: The Abominable Snowmen) or an ivory pyramid. (RP: Downtime) When forcibly summoned to Earth by Anne Travers, and being combined with three sets of physical laws in the Great Kingdom, the Intelligence was an emerald tetrahedron and, because of Travers' meddling, was merged with the god Saraquazel. (MA: Millennial Rites)
At some point it became a swarm of "living snow;" carnivorous, multi-nuclear crystalline organisms which generated a low level telepathic field that could detect and respond to thoughts and memories, mimicking and mirroring whatever it found and feeding off thoughts to learn what it could. This snow-form seemed independent, with some ice crystals forming mindless predatory snowmen and a perfect duplicate of human DNA in ice form, while the main Intelligence remained seperate. Thoughts affecting a critical mass of snow could affect all the snow, leading to the loss of this body when the snow turned to rain.
In its many attempts to achieve form, the Intelligence tried to manifest as ice people based on the human form (DW: The Snowmen) and a slime that glowed brightly with a piercing light. (DW: The Abominable Snowmen) It manifested as a web/fungus/dense fog that consumed anything entering it, which spread through the London Underground and trapped the Doctor's TARDIS. The web could not be destroyed by chemicals, explosives, or flamethrowers. (DW: The Web of Fear) A fourth invasion of Earth had it trying to perpetuate itself in every machine and being, with the whole planet cocooned in web: one mass of thoughts in one global body. (MA: Downtime)
Behind the scenes
- Yog-Sothoth is the name of a fictional deity created by H. P. Lovecraft, first appearing in The Case of Charles Dexter Ward. It is suggested to be the most powerful being of the Cthulhu Mythos and described as existing outside of the space-time continuum.